In tractors of the John Deere mark gearboxes are used whose forward clutch and reverse clutch of the reversing unit are released during operation of each other clutch so that the actuating unit relevant to the clutch is not only switched without pressure but the hydraulic oil located therein is at least partially emptied. In this situation air is located in the actuating unit which prior to switching the corresponding clutch again by means of the actuating unit has to be removed from same so that a hydraulic pressure can be built up in the actuating unit. This means during operation of the tractor that with each change of driving direction a partial emptying of the actuating unit of the one clutch, and a filling of the actuating unit of the other clutch takes place accordingly. The actual release of the one clutch and switching of the other clutch for the other driving direction is a function of the clutch pedal actuated by the operator so that the release of the clutch pedal controls the ventilation and filling of the actuating unit of the clutch which is to be switched. For the operating comfort which the operator experiences it is on the one hand decisive that the actuating unit is ventilated in the shortest possible, precisely determined, time interval so that from the moment of the complete ventilation and after regulating forwards with a low clutch pressure a deliberate and dosed modulation of the clutch pressure can take place by the operator by means of the clutch pedal. Whereas too long a time interval, within which the actuating unit is first completely ventilated, would leave the operator with the feeling of a sluggish gear change, it is on the other hand decisive for the operating comfort which clutch pressure is set with the optimum possible pedal position for the operator immediately after the ventilation of the actuating unit of the clutch which is to be switched. Too high a clutch pressure is hereby particularly detrimental for the operating comfort since, for the operator, too high a clutch pressure manifests itself as an uncontrollable fierce bite of the clutch arrangement during a change of driving direction. Too low a clutch pressure immediately after the ventilation of the actuating unit would likewise appear to the operator as a sluggish gear change. For the best possible satisfactory operating comfort it is thus a matter of determining this time interval of the ventilation and the adjoining clutch pressure immediately following thereon and matching these with one another.
In order to avoid these unacceptable influences on the operating comfort and in view of the fact that each gearbox or each clutch arrangement is subject to tolerances even in high quality production, each gearbox had to run through a calibrating process before starting up a tractor wherein precisely the amount of hydraulic oil which is required for completely ventilating the actuating unit, and precisely the clutch pressure which is applied after ventilation whilst avoiding a fierce bite of the clutch, are determined. Present-day calibrating processes require a load on the output shaft of the clutch arrangement. For certain gearboxes which are installed behind the clutch arrangement, this means that a power flow is switched on from the output shaft of the clutch arrangement via the gearbox and the differential to the drive wheels. In unfavorable cases and those which have to be avoided under all circumstances this power flow can lead to a clear movement of the vehicle during the calibrating process, with the corresponding possibilities of injury in the vicinity of the tractor.